Plasma generating apparatus using microwave

ABSTRACT

The present invention uses as a detector for adjustment measurement, monodyne interference between a microwave for plasma generation and the reflected wave. Analysis of the interference wave that is obtained using monodyne interference allows finding of the phase difference between the incident and the reflected wave and the amplitude of that reflected wave; and controlling of an excited microwave generation/control system based on them allows impedance matching between the excited microwave and the plasma. This method allows very high precision phase detection, and calculation of the characteristics of the plasma based on the detected phase shift. Therefore, it is possible to distinguish noises even in the vicinity of the matched region.

TECHNICAL FIELD

[0001] This invention relates to a plasma generating apparatusperforming impedance matching control for plasma generated by amicrowave and a to-be-applied microwave.

BACKGROUND ART

[0002] In the various manufacturing processes for electric devices,plasma generated by a microwave is used. In order to generate stabilizedplasma, impedance-matching is necessary between a microwave source andplasma, which is a load. Since plasma is unstable as a load, normally itis necessary to control impedance.

[0003] In FIG. 1, plasma generated by a microwave is illustrated. Withinthis plasma generated by the microwave, there is a standing wavegenerated by an incident wave and corresponding reflected wave whenimpedance miss-matches. In plasma impedance-match measurement,conventionally, a standing wave in the above-mentioned plasma isdirectly measured using the Four-Point Prove Method where a probe isinserted in plasma. There are, however, the following problems:

[0004] (1) Plasma is unstable as a microwave circuit load;

[0005] (2) Since the reflected amount of the microwave from plasma issmall, the corresponding detected level at around impedance-matchedregion becomes small, making it impossible to distinguish from noises(S/N ratio worsens); and

[0006] (3) Since the error derived from the poor processing precisionand/or the assembly precision of each probe influences detectionprecision, etc., control precision for impedance matching worsens,making it difficult to provide stabilized plasma.

DISCLOSURE OF THE INVENTION

[0007] The objective of the invention is to perform high precisiondetection of an impedance matching level, thereby controlling ato-be-input microwave for plasma generation.

[0008] In order to achieve the above objective, this invention providesa plasma generator, which controls impedance matching between amicrowave and plasma that occurs due to the said microwave and which ischaracterised by comprising: a microwave generator; an impedanceadjustment unit, which changes the impedance for a microwave given bysaid microwave generator, a plasma generation unit, which receives theimpedance adjusted microwave, generating plasma; a monodyne interferenceunit, which obtains an interference wave using the monodyne interferencebetween the microwave given by the microwave generator and a microwavethat has reflected by the plasma; and control unit, which processes asignal from the monodyne interference unit and provides a matchingcontrol signal; wherein impedane matching between the microwave andplasma is taken by giving the matching control signal from said controlunit to said impedance adjustment unit and controlling that impedance.

[0009] The control unit provides the matching control signal bydetecting the phase of the signal from the monodyne interference unit.

[0010] The monodyne interference unit comprises: a ninty degree coupler,which obtains the sine and the cosine components of the reflectedmicrowave; two mixers, which mix said sine and said cosine componentswith the microwave from the microwave generator, respectively; and alow-pass filter, which removes the high frequency components of themixed sine and the mixed cosine components.

[0011] The impedance adjustment unit performes impedance adjustments byselecting from plural stubs.

BRIEF DESCRIPTION OF DRAWINGS

[0012]FIG. 1 is a drawing explaining the relationship between the plasmaand the incident microwave;

[0013]FIG. 2 is a drawing explaining a basic constitution of thisinvention; and

[0014]FIG. 3 is a drawing showing an embodiment of the presentinvention.

BEST MODE FOR CARRYING OUT THE INVENTION

[0015] Referring to the drawings, the following explains in detail anembodiment of the present invention.

[0016]FIG. 2 shows a configuration of the present invention. The presentinvention, as shown in FIG. 2, utilizes, as a matching level detector,the monodyne interference between a microwave for plasma generation andcorresponding reflected wave. Through analysis of the interference waveresulting from that monodyne interference, the phase difference betweenthe incident and the reflected wave, and the reflected wave amplitudecan be calculated. Control of an exited microwave generation/controlsystem in conformity with them allows impedance matching between thegenerating microwave and the plasma.

[0017] This method especially allows high precision and instantaneousphase detection, and calculation of the characteristics of the plasmabased upon the changes of the detected phases; therefore, even in thevicinity of the matchining region, it is possible to distinguish fromnoises. Since as the name monodyne shows, a plasma generatng microwaveis used as a microwave that generates interferences, even if thefrequency of microwave source fluctuates, the precision cannotdeteriorate.

[0018] FIG.3 is a block diagram showing an example of a specific controlconfigration. In FIG. 3, the signal from the microwave source (not shownin the drawing) is given to the plasma 10, which is a load, via adirectional coupler 140 and three stubs for a maching conditionadjustment 150. The signal reflected from plasma 10 comes viadirectional coupler 140 to a ninty degree hybrid 112, which then dividesit into the sine and the cosine wave components. The reflected signal ismixed with the microwave source signal, which comes from the directionalcoupler 140, by mixers 114 and 116. From the mixed signal, low frequencycomponents are extracted by a low-pass filters 122 and 124. Acalculation control unit 130 compares with a predetermined referencesignal, which is a target value, using the phase shift between theincident and the reflected wave that is calculated from the extractedlow frequency components, as well as the amplitude of the reflected waveobtained at the same time, and then outputs a control signal that playsa role of satisfying impedance match conditions. By automaticallyoperating three stubs for adjustment 150 in conformity with this controlsignal, impedance matching between the generating microwave and plasma10 is taken.

[0019] As described above, the reason why the ninty degree coupler 112extracts the microwave sine components and the cosine components whosephases differ ninty degrees therefrom, respectively, is becausecalculation by the calculation control unit 130 becomes easy, asexplained next.

[0020] Regarding the output of the ninty degree coupler 112, the mixers114 and 116 respectively output the following:

A sin(ω+θ)×sin ω=−(A/2){cos(2ω+θ)−cos θ}

A cos(ω+θ)×sin ω=(A/2){sin(2ω+θ)−sin θ}

[0021] The low-pass filters 122 and 124 remove 2ω components from theseoutput signals, respectively, providing a (A/2) cos θ and a (A/2) sin θsignal. The calculation control unit 130 squares these signals,respectively, and then adds them together, providing the amplitude ofthe reflected wave as shown below:

((A/2)cos θ)²+((A/2)sin θ)² =A ²/4

[0022] It is possible to calculate an accurate phase θ from the (A/2)cos θ signal or the (A/2) sin θ signal using this amplitude.

[0023] By comparing this phase θ and the reference signal (target value)and selecting the stub configured with the transmission lines where, forexample, one of three ends is short-circuited, it is possible to matchthe impedance between the plasma, which is a load, and the to-be-inputmicrowave. The reason why that phase is used here is because a phase isdetectable in the vicinity of the impedance-matched region where thereflected wave becomes feeble as long as an interference occurs.Therefore, high precision control at a high S/N ratio can be carriedout. Furthermore, it is possible to detect the amplitude of thereflected wave and then control it.

[0024] Referencing the above-mentioned circuit, examples where realcontrol for impedance matching for microwave is made are shown in thetable below. Actual example 1 Actual example 2 Frequency of microwave orthe 2.45 GHz 450 MHz like Power 3 kW 5 kW Plasma Ar Ar Matching pointreflection 0.3% or less 0.5% or less coefficient Required adjustmenttime 2 sec. or less 2 sec. or less

[0025] In more greatly mismatching regions, while the Four-Point ProveMethod can provide control within 2 sec., however, in the lowreflectance regions for microwave (in the vicinity of the the matchingregion), this method cannot provide excellent control. As shown in thetable, the monodyne interference method can provide control even in thelatter region.

[0026] The above-mentioned calculation control unit 130 may beconfigured from a microcomputer by A/D converting the input signal to adigital signal.

[0027] Industrial Utilization

[0028] Even though a load such as plasma is unstable, thecharacteristics of the plasma can be measured using monodyneinterference method while upmostly avoiding the influence of noises.Calculation processing of the obtained data using a computer or the likeallows high presicion impedance matching. As a result, the plasmaapparatus can be used for device manufacturing processes .

1. A plasma generator, which controls for impedance matching between amicrowave and plasma that occurs due to the said microwave, comprising:a microwave generator; an impedance adjustment unit, which changes theimpedance for a microwave given from said microwave generator, a plasmageneration unit, which receives the impedance adjusted microwave,generating plasma; a monodyne interference unit, which obtains aninterference wave due to the monodyne interference between the microwavegiven from said microwave generator and a microwave that has reflectedby said plasma; and a control unit, which processes a signal from themonodyne interference unit and provides a matching control signal;wherein impedance matching between the microwave and plasma is taken bygiving the matching control signal from said control unit to saidimpedance adjustment unit and controlling that impedance.
 2. The plasmagenerator according to claim 1, wherein said control unit provides saidmatching control signal by detecting the phase of the signal from themonodyne interference unit.
 3. The plasma generator according to eitherclaim 1 or claim 2, wherein said monodyne interference unit comprises: aninty degree coupler, which obtains the sine and the cosine componentsof the reflected microwave; two mixers, which mix said sine and saidcosine components with the microwave from the microwave generator,respectively; and a low-pass filter, which removes the high frequencycomponents of the mixed sine and the mixed cosine components.
 4. Theplasma generator according to any one of claims 1 through 3, whereinsaid impedance adjustment unit performes an impedance adjustment byselecting from plural stubs.